The use of bitumen (asphalt) compositions in preparing aggregate compositions (including, but not just limited to, bitumen and rock) useful as road paving material is complicated by at least three factors, each of which imposes a serious challenge to providing an acceptable product. First, the bitumen compositions must meet certain performance criteria or specifications in order to be considered useful for road paving. For example, to ensure acceptable performance, state and federal agencies issue specifications for various bitumen applications including specifications for use as road pavement. Current Federal Highway Administration specifications require a bitumen (asphalt) product to meet defined parameters relating to properties such as viscosity, stiffness, penetration, toughness, tenacity and ductility. Each of these parameters defines a critical feature of the bitumen composition, and compositions failing to meet one or more of these parameters will render that composition unacceptable for use as road pavement material.
Conventional bitumen compositions frequently cannot meet all of the requirements of a particular specification simultaneously and, if these specifications are not met, damage to the resulting road may occur, including, but not necessarily limited to, permanent deformation, thermally induced cracking and flexural fatigue. This damage greatly reduces the effective life of paved roads.
In this regard, it has long been recognized that the properties of conventional bitumen compositions may be modified by the addition of other substances, such as polymers. For example, copolymers derived from styrene and conjugated dienes are particularly useful, since these copolymers have good solubility in bitumen compositions and the resulting modified-bitumen compositions have good rheological properties.
It is also known that the stability of polymer-bitumen compositions may be increased by the addition of crosslinking agents (vulcanizing agents) such as sulfur, frequently in the form of elemental sulfur. It is believed that the sulfur chemically couples the polymer and the bitumen through sulfide and/or polysulfide bonds. The addition of extraneous sulfur is used to produce the improved stability, even though bitumens naturally contain varying amounts of native sulfur. Other crosslinking agents are known, such as reactive phenol aldehyde resins, particularly phenol formaldehyde resins.
The second factor complicating the use of bitumen compositions concerns the viscosity stability of such compositions under storage conditions. In this regard, bitumen compositions are frequently stored for up to 7 days or more before being used and, in some cases, the viscosity of the composition may increase so much that the bitumen composition is unusable for its intended purpose. On the other hand, a storage-stable bitumen composition would provide for only minimal viscosity increases and, accordingly, after storage it may still be employed for its intended purpose.
Asphaltic concrete, typically including asphalt and aggregate, asphalt compositions for resurfacing asphaltic concrete, and similar asphalt compositions must exhibit a certain number of specific mechanical properties to enable their use in various fields of application, especially when the asphalts are used as binders for superficial coats (road surfacing), as asphalt emulsions, or in industrial applications. (The term “asphalt” is used herein interchangeably with “bitumen.” Asphaltic concrete is asphalt used as a binder with appropriate aggregate added, typically for use in roadways.) The use of asphalt or asphalt emulsion binders either in maintenance facings as a surface coat or as a very thin bituminous mix, or as a thicker structural layer of bituminous mix in asphaltic concrete, is enhanced if these binders possess the requisite properties such as desirable levels of elasticity and plasticity.
As noted, various polymers have been added to asphalts to improve physical and mechanical performance properties. Polymer-modified asphalts (PMAs) are routinely used in the road construction/maintenance and roofing industries. Conventional asphalts often do not retain sufficient elasticity in use and, also, exhibit a plasticity range that is too narrow for use in many modern applications such as road construction. It is known that the characteristics of road asphalts and the like may be greatly improved by incorporating into them an elastomeric-type polymer which may be one such as butyl, polybutadiene, polyisoprene or polyisobutene rubber, ethylene/vinyl acetate copolymer, polyacrylate, polymethacrylate, polychloroprene, polynorbornene, ethylene/propylene/diene (EPDM) terpolymer and advantageously a random or block copolymer of styrene and a conjugated diene. The modified asphalts thus obtained commonly are referred to variously as bitumen/polymer binders or asphalt/polymer mixes. Modified asphalts and asphalt emulsions typically are produced utilizing styrene/butadiene based polymers, and typically have raised softening point, increased viscoelasticity, enhanced force under strain, enhanced strain recovery, and improved low temperature strain characteristics as compared with non-modified asphalts and asphalt emulsions.
The bituminous binders, even of the bitumen/polymer type, which are presently employed in road applications often do not have the optimum characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration.
Current practice is to add the desired level of a single polymer, sometimes along with a reactant that promotes crosslinking of the polymer molecules until the desired asphalt properties are met. This reactant typically is sulfur in a form suitable for reacting, although as previously noted, other crosslinkers may be used.
However, the cost of the polymer adds significantly to the overall cost of the resulting asphalt/polymer mix. Thus, cost factors weigh in the ability to meet the above criteria for various asphalt mixes. In addition, at increasing levels of polymer concentration, the working viscosity of the asphalt mix becomes excessively great and separation of the asphalt and polymer may occur.
One of the methods commonly utilized in the industry to standardize the measure or degree of compatibility of the rubber with the asphalt is referred to as the compatibility test. The test comprises the mixing of the rubber and asphalt with all the applicable additives, such as the crosslinking agents. The mixture is placed in tubes, usually made of aluminum or similar material, referred to as “cigar tubes” or “toothpaste tubes”. These tubes are about one inch in diameter and about fifteen centimeters deep. The mixture is placed in an oven heated to a temperature of about 162° C. (320° F.). This temperature is representative of the most commonly used asphalt storage temperature. After the required period of time, most commonly twenty-four (24) hours, the tubes are transferred from the oven to a freezer and cooled down to solidify. The tubes are kept in the vertical position. After cooling down, the tubes are cut into thirds; three equal sections. The ring-and-ball softening point of the top one third is compared to the softening point of the bottom section. This test gives an indication of the separation or compatibility of the rubber within the asphalt. The elastomer would have the tendency to separate to the top. The lower the difference in softening point between the top and bottom sections, the more compatible are the rubber and asphalt. In today's environment, most states require a difference of 4° F. (2° C.) or less to consider the asphalt/rubber composition as compatible. Few standards allow a higher difference. The twenty-four hour test is used as a common comparison point. The compatibility test is part of the Standard Specification for Performance Graded Asphalt Binder known as AASHTO MP1, or simply MP1, incorporated by reference herein.
An additional concern in the production of PMA is that the composition of the asphalt component may vary widely and occasionally the conventional methods of making the PMA do not meet the compatibility criteria mentioned because something is sufficiently different about the asphalt that makes it more difficult to incorporate the polymer therein. Since there are many different polymers that may be used and many ways of altering the techniques to make them, sometimes the use of a different polymer may provide better compatibility to the problematic asphalt. Changing the crosslinker system is another way trying to solve compatibility concerns. Despite the known approaches of solving compatibility issues, there is always the need for additional techniques to use when these difficulties arise. Furthermore, all possible permutations of the various components discussed above have not been tried in the art, nor is it obvious to try all such possibilities since it is impossible to predict in advance what the outcome of a particular combination would be, given the complexity of PMA systems and the numerous catalytic and non-catalytic reactions that must occur. The asphalt component is hardly a simple system in and of itself, and is composed a wide variety of compounds.
As may be seen from the above, methods are known to improve the mixing of asphalt and polymer compositions. The needed elements for the commercial success of any such process include keeping the process as simple as possible, reducing the cost of the ingredients, and utilizing available asphalt cuts from a refinery without having to blend in more valuable fractions. In addition, the resulting asphalt composition must meet the above-mentioned environmental concerns and governmental physical properties, such as compatibility. Thus, it is a goal of the industry to maintain or reduce the cost of the polymers and crosslinking agents added to the asphalt without sacrificing any of the other elements and improving the properties of the asphalt and polymer compositions as much as possible.